Apparatus for rectification of an olefin by utilization of refrigeration supplied byliquefied methane



4, 1966 KIYOSHI ICHIHARA 3,

APPARATUS FOR RECTIFICATION OF AN OLEFIN BY UTILIZATION OF REFRIGERATIONSUPPLIED BY LIQUEFIED METHANE Filed Feb. 24, 1964 INVENTOR KWoSHl,lcHnmRH ATTORNEY Patented Oct. 4, 1966 s/s,s 1 Claim. on. 6240) Thepresent invention relates to an improved apparatus adapted forrectification olefin from an olefin-rich gas mixture by the utilizationof refrigeration supplied by liquefied methane gas.

The primary object of the invention is to provide an apparatus of theabove nature which is adapted to reasonably -utilize coldness ofliquefied methane gas for the rectification of olefin.

Generally, liquefied methane gas (L.M.G.) is carried by liquefied gastankers and tank lorries, stored temporarily in liquefied gas storagetanks, and then heated to vaporize by a suitable method for use as fuelor raw materials for chemical industry. In this case, how the coldnessheld in L.M.G. is effectively utilized for other services constitutes animportant factor which determines the economical value in respect ofutilization of L.M.G. A problem which arises in connection with actualutilization of coldness of L.M.G. is that the degree of coldness held inl Nrn. (computed in terms of gaseous body at C. and 760 mm. Hg) ofliquid methane under atmospheric pressure is 88 kcal. at its boilingpoint or 16l.6 C. and 65 kcal. in a temperature range between 161.6 C.and an atmospheric temperature or 20 C., and thus there is not so muchdifference between amounts of coldness held in the portion of latentheat and the portion of sensible heat. However, from a view point ofutilization of such coldness, the coldness in the portion of sensibleheat is available only in an extremely limited range since the coldnessin this specific portion is spread over a wide temperature range of morethan 180, whereas the coldness in the portion of latent heat can beutilized under a definite temperature. In order, therefore, toeffectively utilize the coldness in the sensible heat portion, a heatpump system comprising a compressor and a heat exchanger must beadditionally provided. For this reason, the coldness in the sensibleheat portion close to an atmospheric temperature has not been utilizedheretofore and has been uselessly wasted in most cases.

With a view to effecting reasonable utilization of the coldness held inL.M.G., the invention provides an improved apparatus adapted to utilizesuch coldness of L.M.G. for rectification of olefin (mainly ethylene).

For the purpose of effective utilization of externally suppliedrefrigeration, it is generally most advantageous to utilize it at aportion of a lowest possible temperature. In utilizing the refrigerationby L.M.G. in an olefin rectifying system, it is most desirable that therefrigeration is utilized for the production of a liquid reflux suppliedto the top of a methane extracting tower since a lowest temperatureappears at the top of the methane extracting tower. However, this methodof utilization is not readily applicable for the following reason. Ormore precisely, since so-called L.M.G. is not pure methane and containsa small fraction of such high boiling point components as ethane andpropane, the boiling point of L.M.G. is relatively high or in the orderof 140 C. to 150 C. Therefore, even when L.M.G. is made to vaporize atatmospheric pressure, the methane extracting tower must be operated at aworking pressure of 2.5 atmospheres at the boiling point of 150 C. andmore than 6.5 atmospheres at the boiling point of -140 C. Due to such OFREFRIGERATION SUP- high working pressure, operating temperature becomeshigher as a whole with the result that loss of ethylene increases andgreater wall thickness of the rectifying tower is required. Since, in anapparatus of the kind, an amount of refrigeration required for theproduction of a liquid reflux for a methane extracting tower is usuallyone third or one fourth that for an ethylene tower due to relativeamount of reflux involved in the operation, and a great advantage cannot be derived when L.M.G. is diverted in the form of gas to otherservices for the purpose of making up for a refrigeration loss in theethylene separator. When, therefore, L.M.G. is diverted in the form ofgas to other services in prior systems, the coldness of L.M.G. must bealso utilized for the production of the liquid reflux for the ethylenetower with the result that L.M.G. is thereby gasified. This isapparently disadvantageous in that the coldness of L.M.G. is utilizedfor a service at a relatively high temperature.

The present invention provides an improvement in the prior defectiveutilization of L.M.G. and provides an improved apparatus in which L.M.G.is pressurized by a pump to thereby raise its boiling point up to atemperature of to C. so that a liquid reflux to a methane extractingtower can be produced by the sensible heat of L.M.G. developed up tosuch boiling point.

According to the invention, there is provided an apparatus for therectification of olefin from an olefin-rich gas mixture by theutilization of refrigeration by externally supplied liquefied methanegas comprising a methane extracting tower, an olefin tower, a methanerecirculating cycle including a methane compressor, heat exchangers andan expansion value for recirculating a portion of methane delivered fromsaid methane extract ing tower again into said methane extracting tower,and a pump for conducting at an increased pressure said liquefiedmethane gas to one of the heat exchangers in said methane recirculatingcycle for refrigeration by the sensible heat of said liquefied methanegas, wherein the manner of operation is such that recirculating methaneis first liquefied by the refrigeration by the sensible heat of saidliquefied methane gas and subsequently said liquefied methane gas isused to refrigerate a liquid reflux for said olefin tower.

There are other objects and particularites of the invention which willbecome obvious from the following description with reference to theaccompanying drawing, in which the sole figure is a diagrammaticarrangement of a preferred embodiment of an apparatus according to theinvention. a

In the drawing, reference numeral 1 denotes a methane extracting towerwhich includes therein a reboiler 2. Reference numeral 3 denotes anethylene tower which includes therein a reboiler 4. An ethylenecondenser 5 and a receiver 6 therefor are associated with the ethylenetower 3, and a pump 7 is provided to recirculate a liquid reflux ofethylene from the receiver 6 into the top of the ethylene tower by wayof a conduit 8. The methane extracting tower 1 is connected with theethylene tower 3 by way of a conduit 9. A heat exchanger 10 is providedto effect heat exchange between raw gas and product gases delivered fromthe methane extracting tower 1 and the ethylene tower 3. The heatexchanger 10 is connected with a gas-liquid separator 11 through areboiler 12. A heat exchanger 13 is provided to effect further coolingof noncondensed gas in the gas-liquid separator 11. An expansion valve14 is interposed in a line between the methane extracting tower 1 andthe gas-liquid separator 11, while an expansion valve 15 is interposedin another line connecting the methane extracting tower 1 with thegas-liquid separator 11 through the heat exchanger 13.

a a) According to the invention, a portion of methane delivered from thetop of the methane extracting tower 1 is recirculated through a methanerecirculating cycle 21 which comprises a methane compressor 16;, heatex: changers 1'7, 18 and 19, and an expansion valve 20. Externallysupplied liquefied methane gas (L.M.G.) is pressurized by a pump 22 andsupplied to the heat exchanger 18 in the methane recirculating cycle 21to thereby liquefy the recirculating methane by refrigeration with itssensible heat portion, then L.M.G. is diverted to refrigerate the liquidreflux for the ethylene tower 3.

The apparatus of the invention with the above arrangement operates inthe following manner. Raw gas, from which water, CO and high boilingpoint hydrocarbons including C and more carbon have been eliminated, issupplied into the apparatus by way of a line a and made to pass throughthe heat exchanger it) and the reboiler 12 wherein it is cooled to atemperature of the order of 90 C. by the refrigeration supplied byseparated gases to turn into a condensate. The condensate thereby formedis separated in gas-liquid separator 11, While noncondensed gas is ledby way of a line b into the heat exchanger 13 wherein it is cooled to atemperature of l30 C. to -l40 C. Condensate formed in the heat exchanger13 is led back into the gas-liquid separator ill by way of a line 0,while remaining gas mainly of hydrogen is led through a line a into anexpansion valve 30 wherein it is expanded to a pressure close toatmospheric pressure and lowered in its temperature. The gas is thenmade to flow inversely through the heat exchangers 13 and iii to makeheat exchange with the raw gas, warmed up to a temperature of 43 C. to45 C. and dis charged outwardly of the system.

The condensate collected in the gas-liquid separator 11 is taken outthereof by Way of a line 2 and expanded by the expansion valves 14 and15 to a pressure of 0.5 kg/em. G. The condensate expanded at theexpansion valve 15 is made to flow through the heat exchanger 13,vaporizes by cooling the raw gas and is fed into the methane extractingtower 1, while the condensate expanded at the expansion valve 14 issupplied in its liquid form into the methane extracting tower 1. Methanein the methane extracting tower 1 is led by Way of a line 1 to themethane compressor 16, wherein it is pressurized up to a pressure of theorder of 20 kg./cm. G. Then, methane is led through the heat exchangers17, 18 and 19 for being cooled therein, expanded by the expansion valve2%- to a pressure of 0.5 kg./cm. G and recirculated into the methaneextracting tower 1. In the methane extracting tower l, methane collectsat the top thereof, while ethylene and ethane collect at the bottomthereof. A portion of the methane component is fed back into the methanecompressor 16 by way of the line 1, While the remaining portion orproduct methane is made to flow inversely through the heat exchangers 13and It by way of a line g for heat exchange with the raw gas, warmed upto a temperature of 43 C. to 45 C., and discharged outwardly of thesystem. The ethylene and ethane components separated at the bottom ofthe methane extracting tower i are fed into the ethylene tower 3 by wayof the line 9 under pressure differential.

A manner of rectification in the ethylene tower 3 which is operated at aworking pressure of 0.3 kg./cm. G is similar to that of the methaneextracting tower 1. In the ethylene tower 3, ethylene collects at thetop thereof and ethane at the bottom thereof. The ethylene component isled through a line k, ethylene condenser 5, ethylene receiver 6, and aline it into the heat exchanger in which makes counter flow with the rawgas for heat exchange therewith, is warmed up to a temperature of 43 C.to 45 C, and discharged outwardly of the system. The ethane component isled through a line i into the heat exchanger i in which it makes countertlow with the raw for heat exchange therewith, is

warmed up to a temperature of -43 C. to 45 C., and likewise dischargedoutwardly of the system.

A manner of heating the bottom of the methane extracting tower 1 and theethylene tower 3, and a manner of cooling for the production of theliquid reflux therefor are as follows: L.M.G. is admitted into thesystem by way of a line 1' and pressurized to a pressure of the order ofkg./cm. G by the pump 22. L.M.G. is then fed into the heat exchanger 18for heat exchange with methane pressurized in the methane compressor 16and recirculating through the methane recirculating cycle 21. Byliquefying the recirculating methane, L.M.G. is heated up to atemperature of ll0 C. to -l20 C. and led to the ethylene condenser 5 byway of a line 23. L.M.G. fed into the ethylene condenser 5 is effectiveto liquefy a major portion of ethylene led out of the top of theethylene tower 3 and into the ethylene condenser 5 by way of the line k.(The amount of ethylene to be liquefied corresponds to a required amountof reflux. When product ethylene is to be obtained in the form ofliquid, the entire amount is liquefied. In this case, product ethyleneis directly taken out of the system by way of a line I.) L.M.G. therebyvaporized is led into a heat exchanger 24 by way of a line 211, makesheat exchange with compressed ethylene returned into the heat exchanger24 by Way of a line r after having been used as a reirigeratin medium inother services, is warmed up to a temperature of 45 C. to -50 C., anddischarged outwardly of the system.

Ethylene liquefied in the ethylene condenser 5 flows down into thereceiver 6, and a major portion of liquefied ethylene is led by way of aline 12 into the pump '7, by which it is fed back to the top of theethylene tower 3 by way of the line 8. A portion of liquefied ethyleneis led by Way of a line 0 into a pump by which it is pressurized by anamount more than 3.5 to 4 kg./cm. for example, pressurized up to 10kg/cm. G, warmed up to a certain degree through heat exchange withpressurized liquid ethylene at a relatively high temperature, and fedinto the reboiler 4 of the ethylene tower 3. Liquid ethylene underpressure of 10 l g./cm. G, which is liquefied by heating the ethylenetower 3, is led by way of a line 2 into a pump 27, which compensateswith its head for any pressure loss due to resistance during flow in thepiping and diverts liquid ethylene by way of a line g to anyotherservices requiring refrigeration. Refrigeration temperatureavailable for other services depends upon the working pressure of theethylene tower reboiler 4-, but, in the present case, it is in the orderof C. to C.

Ethylene having been used as a refrigerating medium at other servicesreturns in the form of gas into the system by way of the line 1', ispartly re-liquefied in the heat exchanger 24, and fed into the ethylenetower reboiler 4 and the methane extracting tower reboiler 2 by way of aline s. It will be understood that the rate of admission of ethylenemust be determined from a viewpoint of a heat balance in the system.Ethylene liquefied by heating the methane extracting tower 1 at thereboiler 2 is fed through a line and the heat erchanger 26 into anexpansion valve 29, in which it is expanded to a pressure approximatelyequal to the working pressure of the ethylene tower 3, or 0.3 kg./cm. G,joins the ethylene reflux from the pump 7, and is supplied to theethylene tower 3.

As described in detail in the foregoing, according to the invention, thetemperature of refrigeration available for other services is the boilingpoint of ethylene corresponding to the working pressure of the ethylenetower reboiler 4, and the working pressure can be freely set at anyvalue more than 3.5 to 4 kg/cm. Therefore, the invention providesprominent effects that coldness of L.M.G. can be utilized for theseparation of ethylene and its surplus coldness can be practicallyconveniently utilized in a considerably Wide temperature range, white atthe same time L.M.G. is finally obtained in the form of gas at apressure above kg./cm. G. L.M.G. is mainly utilized by gas producingcompanies and these companies are also producing oil gas. Therefore,application of L.M.G. to the separation of olefin from the oil gas willprovide an extremely great advantage, since L.M.G. obtained in the formof gas after such process can readily be supplied to a town gas system.

Further since refrigeration to provide the required amount of reflux forthe methane extracting tower 1 is only one third or one fourth therefrigeration to provide the required amount of reflux for the ethylenetower 3, it will be apparent that the amount of refrigeration for theproduction of the reflux for the methane extracting tower 1 cansufficiently be provided by the sensible heat contained in the requiredamount of liquefied methane gas (L.M.G.) which is vaporized at atemperature of 110 C. to 120 C. to produce the liquid reflux for theethylene tower 3. Because of a widened temperature range for theutilization of refrigeration by L.M.G. in the methane extracting tower1, the methane recirculating cycle is additionally incorporated in theapparatus of the invention for compressing methane by the methanecompressor 16 for recirculation to the extracting tower 1, as in case ofprior apparatus wherein refrigeration by L.M.G. is not utilized.However, in the apparatus of the invention, boosting up to a pressure ofthe order of 20 atmospheres is only 'suflicient as against 50atmospheres in prior apparatus, since the refrigerating temperature isrelatively low, or in the order of 1l0 C. to -120 C. Therefore, theaddition of such recirculating cycle is not a drawback when comparedwith an increased yield of ethylene owing to low temperature operation,effective utilization of coldness of liquefied methane gas, and otheradvantageous factors.

The invention, further, is very advantageous in that the methanecompressor at a high compression efiiciency can be easily manufacturedin View of the low degree of pressurization of 20 atmospheres, whichcompressor is generally of a labyrinth type structure in order to avoidmixture of oil into compressed gas.

In the invention, a so-called refrigeration cycle is formed and utilizedfor the separation of olefin for the purpose of pressurizing andrecirculating a component such as ethylene having a higher boiling pointthan methane -by means of a pump or compressor, liquefying ethylene bysurplus coldness and utilizing for other services the refrigerationprovided by vaporization of 1iquefied ethylene. Thus, the surpluscoldness in the apparatus working at an elevated temperature level and amajor portion of coldness derived from the sensible heat portion ofL.M.G. can be utilized for refrigeration at a predetermined temperature,and a great degree of utilization of such coldness can thereby beattained.

What is claimed is:

Apparatus for the rectification of olefin from an olefinrich gas mixtureby the utilization of refrigeration by externally supplied liquefiedmethane gas comprising a methane extracting tower, an olefin tower, amethane recirculating cycle including a methane compressor, heatexchangers and an expansion valve for recirculating a portion of methanedelivered from said methane extracting tower again into said methaneextracting tower, and a pump for conducting at an increased pressuresaid liquefied methane gas to one of the heat exchangers in said methanerecirculating cycle for refrigeration by the sensible heat of saidliquefied methane gas, wherein the manner of operation is such thatrecirculating methane is first liquefied by the refrigeration by thesensible heat of said liquefied methane gas and subsequently saidliquefied methane gas is used to refrigerate a liquid reflux for saidolefin tower.

References Cited by the Examiner UNITED STATES PATENTS 3,213,631 11/1965Kniel 62-26 X FOREIGN PATENTS 1,233,881 10/1960 France.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

